Wind/Wave PropagationDir

Hello

I would appreciate if you could help me to understand the sign criteria for the propagation direction of the waves and wind using the input values WaveDir from HydroDyn module and PropagationDir from Inflowind.

The definitions on the user guides say:

WaveDir is the mean wave propagation heading direction (in degrees), and must be in the range (-180,180].A heading of 0 corresponds to wave propagation in the positive X-axis direction. And a heading of 90 corresponds to wave propagation in the positive Y-axis direction.

PropagationDir: Direction of wind propagation [degrees]. This input parameter is used to rotate the entire wind field for all wind file types. The rotation angle is specified as a meteorological rotation angle about the Zi-axis (rotation from alignment with positive Xi towards negative Yi). This is a clockwise rotation when looking from above towards the surface of the ground or ocean.

I have drawn and attached what I understand from these definitions but I am not sure if I am understanding this sign convention correctly. In the image you will see that both waves and wind rotation is positive in the same direction but I am not sure if wind rotation sign is opposite. Would you mind taking a look at it and confirming if this is correct or not. If not I would appreciate a similar kind of drawing with the correct sign convention.

Many thanks

Gustavo

Dear Gustavo,

Your image for the wave propagation direction in HydroDyn (WaveDir) is correct. But your image for the wind propagation direction in InflowWind (PropagationDir) is reversed. WaveDir is defined counter-clockwise when looking from above; PropagationDir is defined clockwise when looking from above.

I hope that helps.

Best regards,

Dear Jason

First, thank for your prompt reply. It is clear now and I can keep on running simulations confidently.

I had the feeling that Wind propagation sign convention was reversed as you mentioned but I was not 100% sure about it. I was wondering why making it opposite to the wave sign convention and that made me doubt.

Best Regards

Gustavo

Dear Gustavo,

The wind propagation direction follows standard meteorological convention, whereas most other FAST inputs/outputs follow the “right-hand rule” for rotation about an axis.

Best regards,

Dear Jason

Thank you for the clarifying this point. It makes complete sense.

Regards

Gustavo

Hello,

I was wondering if someone can please clarify the directions for wind and waves. Is it true that PropagationDir is specified as the direction that the wind is coming from, but WaveDir is the direction that the waves are heading towards?
I understand that the wind direction is specified looking clockwise from above and wave direction is anti-clockwise, but is the 0 degrees direction aligned for both coordinate systems?

Thanks,

Rachael

Dear Rachael,

I would say that PropagationDir in InflowWind defines the wind direction and WaveDir in HydroDyn defines the wave direction, but–as you said, PropagationDir is defined clockwise from above and WaveDir is defined anti-clockwise from above. Zero degrees for both means that the wind and waves propagate along the +X axis of the global inertial frame coordinate system.

Best regards,

Dear Jason,

Thank you for your response. Sorry for the stupid question, but does that mean that if the +X axis is pointing south, then zero degrees for both would mean that the wind and waves are both propagating from north to south?

Thanks,
Rachael

Dear Rachael,

Yes, that is correct. That said, FAST / OpenFAST does not explicitly ues the compass direction, and so, has no knowledge of north/south/east/west.

Best regards,